The present invention relates to a master cylinder for hydraulically actuated brakes for automotive vehicles.
There are already known various constructions of master cylinders of the above type. In one known construction, the master cylinder includes at least one master cylinder piston with a piston return spring anchored by a central clamping member and a valve body which is slidable relative to the master cylinder piston and opens a compensating port in the master cylinder piston when the piston return spring is relieved and which carries at its end face close to the compensating port an elastic sealing element whose outer diameter is greater than the diameter of the compensating port.
The master cylinder piston is guided in a cylinder bore of a housing in an axially slidable and sealed relationship. An end face of the piston that is close to the brake pedal delimits a chamber that is connected to a return reservoir. The end face of the master cylinder piston that is remote from the pedal delimits a working chamber which is adapted to be pressurized upon brake pedal actuation. The chambers on the two sides of the master cylinder piston are interconnected when the brake pedal is released, that is, when the brake is not actuated, by a compensating port or passage which centrically opens onto the end surface of the master cylinder piston which faces to the working chamber. In alignment with this compensating port, there is accommodated in the working chamber a valve body slidable relative to the master cylinder and carrying an elastic sealing element by which the compensating port can be closed upon brake application.
The inactive position of the arrangement is brought about by a piston return spring acting on the bottom of the master cylinder and on the master cylinder piston, as well as by a valve closure spring acting on the valve body. Besides, a clamping member is shaped at the valve body, which keeps the elastic sealing element at a maximum distance from the compensating port when the brake is released.
When an actuating force is transmitted via a brake pedal to the master cylinder piston upon brake application, first the piston return spring which acts on the master cylinder piston and on the bottom of the master cylinder is compressed, as a result of which the valve closure spring is relieved by the same amount so that the elastic sealing element moves in the direction toward the compensating port. When the actuating force is of a sufficiently great magnitude, the sealing element contacts the piston around the compensating port and thus causes interruption of the connection between the working chamber of the master cylinder and the return reservoir. Upon further displacement of the master cylinder piston, the working chamber of the master cylinder that is remote from the pedal will be pressurized, and an actuation of the wheel brakes connected to this working chamber is initiated thereby. During the brake-release action, the master cylinder piston will be gradually returned to such an extent that the elastic sealing element eventually lifts from the surface surrounding the compensating port, and both the working chamber of the master cylinder and the wheel brakes become unpressurized.
Master cylinders of the above type incorporating so-called central valves have the advantage, in comparison with sleeve-sealed master cylinders, that there is no need for a so-called breather bore which has to be traveled over regularly when the brake is applied. When traveling over such breather bores, the sleeve seals are subjected to relatively high wear. In particular in a combination with an anti-skid control unit, it is possible that very high pressures develop in the working chamber already in the case of small actuating travels of the master cylinder pistons. It cannot be excluded under such circumstances that the sleeve seal is pressed by the high pressure differences acting on it into the breather bore and is rendered inoperative thereby.
However, it has been established even in connection with the master cylinders with central valves as described above that the elastic sealing element is similarly pressed into the central compensating port under specific operating conditions. If this is a frequent occurrence, even in this case the sealing element may become brittle as a result of the large extent of deformations or may be sheared off at the edges of the compensating port, respectively. As a result, the sealing effect of the elastic sealing element is considerably impaired.